Catch APF description up with recent developments

Primarily the change, in release 1.22, of how configuration objects
are maintained.

Also describe the new priority level.

Also move the sections on defaults and health check configuration to
follow the description of configuration objects.

content/en/examples/priority-and-fairness/health-for-strangers.yaml

@@ -1,4 +1,4 @@
-apiVersion: flowcontrol.apiserver.k8s.io/v1beta1
+apiVersion: flowcontrol.apiserver.k8s.io/v1beta2
 kind: FlowSchema
 metadata:
   name: health-for-strangers